Light induced plating of metals on silicon photovoltaic cells

ABSTRACT

A method and composition for plating metal contacts on photovoltaic solar cells is described. The cell is immersed in an aqueous bath containing platable metal ions and a solubilizing agent for aluminum or aluminum alloy ions from the back side of the solar cell. The cell is then exposed to light, causing the two sides of the cell to become oppositely charged. The metal ions are plated without requiring an external electrical contact.

This application is a continuation in part of U.S. application Ser. No.12/545,547, now U.S. Pat. No. 8,722,142, filed on Aug. 28, 2009.

FIELD OF THE INVENTION

The present invention relates generally to a method of light inducedplating of metal contacts on photovoltaic cells, including silicon solarcells.

BACKGROUND OF THE INVENTION

Solar cells are photovoltaic cells or modules, which convert sunlightdirectly into electricity. Photovoltaic (PV) cells are made ofsemiconductor materials, most commonly silicon. When light (ultraviolet,visible and infrared radiation) strikes the cell, a certain portion ofit is absorbed within the semiconductor material, such that the energyof the absorbed light is transferred to the semiconductor and anelectrical current is produced. By placing metal contacts on the top andbottom of the PV cell, the current can be drawn off to use externally.The current, together with the cell's voltage, defines the wattage thatthe solar cell can produce.

A typical semiconductor photovoltaic cell comprises a large area p-njunction, where absorption of light results in the creation ofelectron-hole pairs. The electrons and holes migrate to opposite sidesof the junction such that excess negative charge accumulates on then-doped side and excess positive charge accumulates on the p-doped side.In order for the current to be collected for power generation,electrical contact of both sides of the pn junction to an externalcircuit must be made. The contacts typically consist of two metallicpatterns, with one on each side, in ohmic contact with the semiconductordevice. The ideal contacting pattern will have high conductivity inorder to minimize resistive losses, good electrical contact to thesubstrate in order to efficiently collect current, and high adhesion toensure mechanical stability. The metal pattern on the cell front-side,defined as the side of the cell which is exposed to incident light, isdesigned to provide a low resistance path for collecting currentgenerated at any location on the surface of the cell and,simultaneously, to minimize the amount of incident radiation interceptedby the metal and thus lost for current generating purposes.

Silicon, especially in its crystalline form, is a common material usedfor producing solar cells. Most solar cells are made from crystallinesilicon, doped with boron and phosphorus to produce a p-type/n-typejunction. Polycrystalline silicon can be used in solar cell fabricationto cut manufacturing costs, although the resulting cells may not be asefficient as single crystal silicon cells. Amorphous silicon, which hasno crystalline structure, may also used, again in an attempt to reduceproduction costs. Other materials used in solar cell fabricated includegallium arsenide, copper indium diselenide and cadmium telluride.

A typical arrangement of a silicon solar cell comprises as follows:

-   -   (a) a back contact usually comprising a layer of aluminum or        aluminum alloy and busbars comprised of silver or a        silver-aluminum alloy;    -   (b) a P-type Si;    -   (c) an N-type Si;    -   (d) an antireflective coating on the front-side of the cell;    -   (e) a front contact usually comprising a metallic grid of        fingers and busbars; and    -   (f) a cover glass.

Because silicon is extremely reflective, an antireflective coating istypically applied to the top of the cell to reduce reflection losses. Aglass cover plate is then typically applied over the antireflectivelayer to protect the cell from the elements.

Conventional solar cells may be made using crystalline silicon wafers.The Si wafer starts as a p-type semiconductor with a boron dopant. Tobetter capture light, the wafer may be texturized with hydroxide ornitric/hydrofluoric acids so that light is obliquely reflected into thesilicon. The p-n junction is formed by diffusion of an n-type dopant,typically phosphorus, using vapor deposition or diffusion. A layer ofdielectric is applied, typically silicon nitride or silicon oxide, tothe front-side; this layer serves as both a surface passivation layerand an anti-reflective coating (ARC).

In one standard process of silicon solar cell fabrication, the frontside of the silicon wafer is coated with an anti-reflective passivationlayer, which typically comprises silicon nitride. This silicon nitridelayer serves the dual purpose of maximizing the percentage of lightabsorbed by the cell (not reflected), as well as passivating thesurface, which prevents electron-hole recombination at the surface andthus increases cell efficiency. A different effect is typically employedon the cell back surface to minimize electron-hole recombination. A“back surface field” (BSF) is achieved by making a so-called p⁺-dopedlayer in the silicon close to the rear surface, where the p⁺-doped layercontains a higher concentration of p-dopant than the bulk p-dopedsubstrate. This creates an electric field close to the interface whichprovides a barrier to minority carrier (electron) flow to the rearsurface. Any p-dopant such as aluminum or boron may be used. Typicallyaluminum or aluminum alloy is deposited on the back surface and fired athigh temperature in order to diffuse p-dopant into the silicon. Thealuminum or aluminum alloy may be deposited by screen printing or vapordeposition. Typically, in a standard process for silicon solar cellfabrication, solderable busbars comprising silver or silver-aluminumpaste are screen printed on the rear surface, then aluminum paste isscreen printed over the entire back surface except areas covered by thebusbars, then fired to remove solvent binders, harden the paste layers,and diffuse the aluminum p-dopant into the silicon.

The solar cell contacts must be formed, whereby a full area metalcontact is made on the back surface comprising the p⁺-doped BSF andsolderable busbars as described above, and a grid-like metal contactmade up of fine “fingers” and larger “busbars” is formed on the frontsurface, typically formed by screen printing silver paste into a patternof said fingers and busbars, then firing at high temperature to removesolvent binders, harden the pattern, and form ohmic contact to the cell.After the solar cell conductors are formed, multiple solar cells arethen interconnected in series (and/or parallel) by flat wires or metalribbons, and assembled into modules or “solar panels.” The finishedsolar panel product typically has a sheet of tempered glass on the frontand a polymer encapsulation on the back to protect it from theenvironment.

Silicon is the most commonly used material for solar cell panelmanufacturing. FIG. 1 shows the front side 10 having front side metalbusbars 12 and metal lines 14 and the backside 20 having backside metalbusbars 22 of a typical silicon solar cell. The metal busbars 12 andmetal lines 14 on the front side of the solar cell preferably comprise aprinted silver paste which is plated upon with the plating compositionand method of this invention. The backside metal busbars 22 preferablycomprise either silver paste or an aluminum-silver paste in contact withsilicon. The remainder of the cell back side is preferably covered by afired aluminum layer 38.

FIG. 2 shows a cross-sectional view of a typical silicon solar cellhaving an anti-reflective coating layer 32, an n-doped silicon layer 34and a p-doped silicon layer 36. The silicon may be single crystalline ormulticrystalline silicon, by way of example and not limitation. Themetal lines 14 on the front side 10 collect the light-induced current.The front side busbars 12 collect current from the multiple metal lines14 or “fingers.” The backside 20 of the cell typically has a set ofbusbars 22 similar to the front side; however, the backside 20 does notneed to allow for transmission of light. The front side busbars 12 andbackside busbars 22 allow for the connection of cells in series formodules. The backside busbars 22 contact the silicon substrate. Theremainder of the back side is covered with a layer 38 of aluminum oraluminum alloy. The layer of aluminum or aluminum alloy 38 is generallyapplied by printing an aluminum or aluminum alloy paste on the backsideof the solar cell and then baking the layer.

Competing factors must be considered in designing the front side metalpattern. The front side of the device must allow transmission of lightso the metal traces should cover the smallest possible area in order tominimize shading losses. On the other hand, efficient current collectionfavors the coverage of the largest possible surface area since the sheetresistance of the front side may be relatively high (about 50 to 100Ωper square), leading to resistive losses if the coverage is too low.

A variety of methods may be used to form the front metal pattern,including screen printing of conductive paste, ink jetting, andelectroplating onto a seed layer. One commonly used method is screenprinting of a silver paste containing a glass frit, followed by a firingstep at about 800° C. during which the paste burns through theanti-reflective coating, if present, forming a grid of metal paste linesand busbars. While this method provides conductive patterns withreasonably good electrical contact, conductivity and adhesion,performance could be further improved by the deposition of additionalmetal onto the grid, since the post-fired paste necessarily containsvoids and non-metallic filler.

In another method used to form a front side conductive pattern, metal isdeposited from a solution of soluble metal ions onto a pattern of linesand busbars formed in the anti-reflective coating. A variety of methodsmay be used to form the pattern, such as photolithography followed byetching, mechanical scribing, or laser imaging. Such a method isdescribed in International Publication No. WO 2005/083799.

Deposition of a metal from a solution of soluble metal ions occurs by anelectrochemical mechanism, where oxidation and reduction reactions takeplace. Defined broadly, there are three different mechanisms fordepositing metal on a substrate from solution:

-   (1) Displacement, also known as galvanic, deposition is where    deposition of a metal on a less noble metal substrate is accompanied    by transfer of electrons from the less noble to the more noble    metal, resulting in deposition of the more noble metal and    dissolution of the less noble metal substrate. This method may be    limited in that the deposit may be limited in thickness since    deposition will normally stop when the less noble substrate is    completely covered. Also a portion of the substrate will be    consumed. Also, the deposited metal layer may be non-continuous    since deposition of the more noble metal is accompanied by    dissolution of the underlying less noble metal, leading to poor    conductivity and adhesion.-   (2) Electrolytic plating is where oxidation and reduction are    induced, by means of an external source of electrical current. This    method provides dense, high quality metal layers with fast    deposition rates that are not limited in thickness. However, an    electrical connection must be made to the substrate, with an anode    present in the bath to complete the electrical circuit. Making good    electrical contact with the cells may be problematic since it may    result in breakage of the fragile silicon substrate.-   (3) Autocatalytic, also known as electroless plating, deposition is    where reduction of the metal ions is accomplished chemically by    inclusion of a reducing agent in solution, where deposition only    takes place on catalytically active surfaces. This method eliminates    the need for electrical contact and an external power source.    However, in practice this method suffers several drawbacks. Firstly,    the process may be difficult to control since the solution is    inherently thermodynamically unstable; spontaneous decomposition    with precipitation of metal may occur unless great care is taken to    optimize the system. This in turn limits deposition rates which may    be very slow. In particular, autocatalytic silver plating solutions    are well known in the art to be highly unstable.

To solve some of these problems, the prior art has suggested variousmethods of electroplating on photovoltaic devices utilizing, forexample, light-induced voltage to effect metal deposition.

U.S. Pat. No. 4,144,139 to Durkee, the subject matter of which is hereinincorporated in its entirety, describes a method for plating electricalcontacts onto the surface of a solar cell by immersion of the cell in anelectrolyte solution containing metal ions and exposing the surface ofthe solar cell to light such that plating of metal occurs on the anodesurface of the device. The back (anodic) side is covered by a thicksacrificial layer of silver, such that silver dissolves from the anodicbackside and deposits on the cathodic front side when the device isirradiated. A cyanide-containing silver plating solution is described.Although cyanide-containing silver electrolytes are well known to yieldexcellent plating results, the use of cyanide is not preferred due tosafety as well as environmental considerations.

U.S. Pat. No. 4,251,327 to Grenon, the subject matter of which is hereinincorporated by reference in its entirety, describes an electrolyticmethod for plating similar to that described in U.S. Pat. No. 4,144,139.In addition, this patent describes an arrangement where the anodicbackside of the device is attached to the negative terminal of a DCpower supply and the positive terminal of the power supply is attachedto a silver electrode in the plating solution, such that when the cellis irradiated with light and the current is adjusted appropriately,neither deposition nor corrosion occurs on the backside. Again, acyanide-containing silver plating solution is used. This arrangement isshown in FIG. 3. A negative aspect of this method is the requirement ofattachment of an electrode to the cell, which may result in breakage offragile silicon substrates.

U.S. Pat. No. 5,882,435 to Holdermann, the subject matter of which isherein incorporated by reference in its entirety, describes a processwhere a printed metallic front side pattern on a photovoltaic cell isreinforced by photo-induced deposition of a metal such as copper orsilver. The back (anodic) side includes a printed sacrificial metalpaste such that charge neutrality is maintained by dissolving of metalfrom the back side concurrent with deposition on the front side when thedevice is irradiated.

U.S. Patent Publication No. 2008/0035489 to Allardyce, the subjectmatter of which is herein incorporated by reference in its entirety,describes a method of plating electrical contacts on photovoltaicdevices where the device is exposed to light while immersed in a silverplating solution comprising silver ions, at least one of anitro-containing containing compound, a surfactant, an amino compoundand at least one of either an amino acid or sulfonic acid. However, thiselectrolytic “light induced plating method” for metallizing photovoltaicdevices is the same or similar to the method described in the previouslydescribed U.S. Pat. No. 4,251,327 patent and suffers from the samedeficiency, namely the requirement of attachment to the cell of anelectrode connected to a current source and anode.

U.S. Patent Publication No. 2007/0151863 to Morrissey, the subjectmatter of which is herein incorporated by reference in its entirety,describes a non-cyanide silver electroplating composition comprisingsilver in the form of a complex with hydantoin or substituted hydantoin,an electrolyte, and 2,2′-dipyridyl.

U.S. Pat. No. 5,601,696 to Asakawa, the subject matter of which isherein incorporated by reference in its entirety, describes acyanide-free silver electroplating bath containing silver in the form ofa complex with hydantoin or substituted hydantoin.

U.S. Pat. No. 4,126,524 to Hradil et al., the subject matter of which isherein incorporated by reference in its entirety, discloses cyanide-freesilver electroplating solutions containing silver complexed with imidesof organic dicarboxylic acids, such as succinimide.

U.S. Pat. No. 4,246,077 to Hradil et al., the subject matter of which isherein incorporated by reference in its entirety, describes cyanide-freesilver electroplating solutions containing silver complexed withpyrrolidine-2,5-dione (succinimide) or 3-pyrroline-2,5-dione(maleimide).

U.S. Pat. No. 5,322,553 to Mandich et al. the subject matter of which isherein incorporated by reference in its entirety describes acyanide-free electroless silver plating solution comprising a silvercation, thiosulfate, and sulfite. This patent claims a plating rate andsolution stability superior to conventional silver plating solutions.

Electroless silver plating also suffers from several deficiencies. Forexample, the baths are well known to be highly unstable withdecomposition occurring readily, causing loss of silver due toprecipitation, limited bath life, and un-wanted deposition on surfaceregions that should remain free of metal. Also, plating rates aregenerally very slow under conditions necessary for suitable stability.

Faster plating rates can be obtained by electroplating, includinglight-induced plating as described in the prior art, in which anexternal power supply provides current to the devices. However, theattachment of an electrical connection can be problematic in that it canresult in breakage of fragile silicon solar cells.

Therefore, it would be desirable to provide a plating method that iscapable of the faster plating rates realized by electroplating withoutresulting in breakage of the silicon solar cells from the attachment ofan electrical connection and that also minimizes the noted deficienciesof electroless silver plating.

The present invention addresses these deficiencies by using thespecified plating solution and method. The improved plating solution ofthe invention is activated by light when used to plate metal onphotovoltaic cells. The plating solution does not contain cyanide. Noelectrical contact with the device is necessary.

SUMMARY OF THE INVENTION

It is well known that aluminum, while being a non-noble metal, normallyremains well passivated by aluminum oxide except at very acidic oralkaline pH's in aqueous solution, since aluminum oxide can be dissolvedby acids and bases. Moreover, it may undergo fast, uncontrollablecorrosion by reaction with water in acidic or alkaline solution.Therefore, a dilemma is presented in that a neutral or near-neutral pHis desirable in order to avoid uncontrolled corrosion, but aluminumnormally remains well-passivated at neutral or near-neutral pH. Theinventors have found that inclusion of certain reagents for assisting inthe dissolution of the metal on the back surface of the photovoltaiccell can yield steady, controllable dissolution at a more neutral pH.

It is an object of the present invention to provide a method andcompositions for plating metallic conductors onto photovoltaic devicesby a contact-free, light-assisted autocatalytic or displacement process.

It is another object of the present invention to provide a method andcompositions for plating metallic conductors onto photovoltaic devicesby an autocatalytic or displacement process that is activated by light.

To that end, in one embodiment, the present invention relates generallyto a composition for plating metal contacts on a photovoltaic solarcell, the composition comprising:

a) a source of soluble silver ions; and

b) an agent for assisting in the dissolution of the metal on the backsurface of the photovoltaic solar cell (usually aluminum or aluminumalloys).

In another embodiment, the present invention relates generally to amethod of metallizing a photovoltaic solar cell to deposit a thick layerof metal thereon, said photovoltaic solar cell having a front side and abackside, wherein said backside comprises a layer of a metal selectedfrom the group consisting of aluminum and aluminum alloys and said frontside having a metallic pattern thereon, the method comprising the stepsof:

-   -   a) contacting the photovoltaic solar cell with an light induced        plating composition comprising:        -   i) a source of soluble silver ions; and        -   ii) an agent that solubilizes metal ions dissolved from the            metal on the backside; and thereafter    -   b) illuminating the photovoltaic solar cell with radiant energy        from a light source,

wherein the front side and backside of the solar cell become oppositelycharged and metal ions from the plating solution are plated onto themetallic pattern on the frontside of the solar cell, whereby a layer ofmetal is deposited thereon and wherein ions of the metal layer on thebackside are dissolved into the plating solution.

Using this method any metal may be deposited on the front side of thesolar cell from an aqueous solution of its ions provided that themetal's reduction potential is greater than that of water. Preferredmetals include copper and silver, especially silver, due to their highconductivity.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a front side and a backside of a silicon solar cell.

FIG. 2 shows a cross-sectional view of a silicon solar cell.

FIG. 3 shows an embodiment of a prior art light-induced electrolyticplating process.

FIG. 4 shows an embodiment of a light induced plating process inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally to a method and compositions forplating metallic conductors onto photovoltaic devices by a process whichis activated by light and does not require electrical contacts with thedevice.

In one embodiment, the light induced plating composition of the presentinvention includes:

a) a source of soluble silver ions; and

b) an agent for solubilizing metal ions comprising aluminum and alloyconstituents with aluminum.

Almost any compound containing silver (I) can be used in the compositionof the invention. The source of soluble silver ions may be silver oxide,silver nitrate, silver methanesulfonate, silver acetate, silver sulfate,silver citrate, or any other silver salt, by way of example and notlimitation. In one embodiment, the source of soluble silver ions ispreferably silver sulfate, silver acetate, or silver methanesulfonate.In a preferred embodiment the source of soluble silver ions is presentin the light induced plating composition of the invention at aconcentration yielding about 1 to 35 grams/liter of silver metal.

While not wishing to be bound by theory, the inventors believe that theplating reaction on the front side of the solar cell begins as areduction of metal cations driven by the electrical potential caused byexposure of the solar cell to light, where negative charges accumulateon the front (cathodic) side of the cell and positive charges (“holes”)accumulate on the back (anodic) side. As the plating reaction proceeds,absorption of photons continues to generate electron-hole pairs in thebulk silicon; because of the voltage caused by the p-n diode, negativecharges migrate to the front side of the cell and positive chargesmigrate to the back side. The positive charges, or holes, areneutralized by the transfer of electrons from metallic aluminum on theback side of the cell, accompanied by dissolution of aluminum ions,driving the plating reaction forward. Thus for the smooth continuationof the plating reaction it is important for the plating solution tocomprise an agent that solubilizes aluminum ions (and ions of anyaluminum alloy constituents) from the back side of the solar cellpreferentially to other metals on the back side, especially silver whichis typically present in the form of hardened silver paste formingbusbars.

The plating composition should include a compound which is capable ofsolubilizing (either directly or through chelation) aluminum ionsdissolved from the back side of the solar cell. The solubilizingcompound should be able to solubilize aluminum preferentially oversilver. These solubilizing compounds can include glyoxylic acid,glycolic acid, lactic acid or any alpha- or ortho-hydroxycarboxylicacids, aliphatic dicarboxylic acids, hydroxy-polycarboxylic acids suchas citric acid, tartaric acid, malic acid or any otherhydroxy-polycarboxylic acids. Preferred solubilizing compounds arecitric acid, tartaric acid, and glyoxylic acid. The concentration of thesolubilizing compound in the plating composition is not critical, butcan preferably range from 1 to 150 grams/liter, but is most preferablyfrom 10 to 30 grams/liter.

A reducing agent may be added to the plating composition but it is notrequired. Without wishing to be bound by theory, the inventors believethe plating reaction may be assisted by a reducing agent through directtransfer of electrons from the reducing agent to either the front sideor back side of the cell. If used the reducing agent may includeformaldehyde, glucose, dextrose, glyoxal, sugar inverted by nitric acid,hydrazine or hydrazine sulfate, aldonic acids, aldonic lactones,tartrate salts (also known as “Rochelle's salts”), cobalt ions, sulfidesalts, sulfite salts, thiosulfate salts, hypophosphite salts,borohydride salts, dimethylamine or other alkylamine borane,hydrazineborane, cyanoborohydride salts, by way of example and notlimitations. Other reducing agents known in the art would also be usablein the present invention. In one embodiment, the reducing agent is aglyoxylic acid salt, a Rochelle salt, or glyoxal. In a preferredembodiment, the reducing agent is present in the light induced platingcomposition of the invention at a concentration of about 1 to about 60grams/liter, but is most preferably from 15 to 20 grams/liter.

An optional complexing agent may be added to solubilize and stabilizethe silver cation and sequester metallic impurities that may be present.Silver complexors include cyanide, succinimide or substitutedsuccinimides, hydantoin or substituted hydantoins, uracil, thiosulfates,ammonia or other amines, by way of example and not limitation. Aspreviously noted, the use of cyanide is undesirable due to safety andenvironmental concerns. The use of ammonia is also undesirable due toits volatility and its ability to form explosive silver salts upondrying. In a preferred embodiment, the complexing agent is hydantoin or5,5-dimethylhydantoin, or other substituted hydantoin. If used, thecomplexing agent may be present in the light induced plating compositionat a concentration of about 20 to about 150 grams/liter, more preferablyabout 40-80 grains/liter.

Finally, the composition of the invention may also include varioussurface active agents, grain refiners, brighteners, or surfactants. Forexample, polyethyleneimine, polyethylene glycol, 2,2′-dipyridyl,histidine, cysteine, imidazole or imidazole derivatives, may be added tothe composition of the invention by way of example and not limitation. Apreferred brightening agent is histidine which the inventors have foundto be beneficial to producing a white silver deposited layer.

The pH of the solution is preferably adjusted to between about 7.0 and12.0, more preferably between about 8.8 to 9.5, using a suitable pHadjuster. Potassium hydroxide, sodium hydroxide, sulfuric acid, oracetic acid, by way of example and not limitation, may be used to adjustthe pH of the solution.

As illustrated in FIG. 4, the present invention relates generally to amethod of metallizing a photovoltaic solar cell to deposit a layer ofmetal thereon, said photovoltaic solar cell having a front side and abackside wherein said back side comprises a layer of a metal selectedfrom the group consisting of aluminum and aluminum alloys, and saidfront side having a metallic pattern thereon, the method comprising thesteps of:

-   -   a) contacting the photovoltaic solar cell with an light induced        plating composition comprising:        -   i) a source of soluble silver ions; and        -   ii) an agent that solubilities metal ions dissolved from the            metal on the back side; and thereafter    -   b) illuminating the photovoltaic solar cell with radiant energy        from a light source,

wherein the front side and backside of the solar cell become oppositelycharged and metal ions from the light induced plating solution areplated onto the metallic pattern on the frontside of the solar cell,whereby a layer of metal is deposited thereon, and wherein ions of themetal a on the back side are dissolved into the light induced platingsolution.

As discussed above, the metallic pattern on the front side of the solarcell generally comprises a plurality of current collection lines andbusbars.

The light source of the invention is positioned to illuminate thephotovoltaic solar cell with radiant energy. Various light sources canbe used in the practice of the invention, including, for example halogenlamps, incandescent lamps, fluorescent lamps, light emitting diodes, ormercury lamps. Any intensity or wavelength of light can be used,provided that the wavelength is equal to or less than about 1150 nmwhich corresponds to the band gap energy of silicon.

The step of contacting the photovoltaic cell with the light inducedplating composition typically comprises immersing the photovoltaic cellin the plating composition. In the practice of this invention noelectrical contact to an external power source is required.

While not wishing to be bound by theory, the inventors believe thatthere are two possible mechanisms for light-activated plating byreduction of a metal ion on the cathode. Firstly, electrons may bedonated to a metal cation directly from the cathode causing thedeposition of the metal atoms; since positive charges accumulate on theopposite anodic side, electrons must be supplied to this opposite side,either by oxidation of a chemical reducer present in the platingsolution, or oxidation and dissolution of metal from the opposite anodicside. Alternatively, the cathode may catalyze the donation of anelectron from the chemical reducer to the metal, resulting in thedeposition of the metal ion on the cathode. A combination of these twomechanisms may also be occurring. The result of either mechanism is thesame, namely, light-induced deposition of metal occurs selectively ontoa metal cathode without the need for attachment of external electricalcontacts to the cell.

EXAMPLES Example 1

A plating solution was made as follows:

72.6 grams/liter 5,5-dimethylhydantoin 20.2 grams/liter silver acetate14.0 grams/liter potassium methanesulfonate 30.0 grams/liter sodiumpotassium tartrate (“Rochelle salt”) KOH was added to make pH = 9.1

Example 2

A plating solution was made as follows:

72.6 grams/liter 5,5-dimethylhydantoin 20.2 grams/liter silver acetate18.2 grams/liter glyoxylic acid hydrate 72.7 grams/liter potassiummethane sulfonate NaOH was added to make pH = 9.2

Example 3

A plating solution was made as follows:

48.0 grams/liter hydantoin 10.3 grams/liter methanesulfonic acid 32.0grams/liter boric acid 28.7 grams/liter silver methanesulfonate 54.0grams/liter glyoxylic acid hydrate KOH was added to make pH = 8.8

Comparative Example 1

For comparison, a plating solution was made without a solubilizer orreducer as follows:

72.6 grams/liter 5,5′-dimethylhydantoin 20.2 grams/liter silver acetateNaOH was added to make pH = 9.1

Solar cells as illustrated in FIGS. 1 and 2 were plated with thesesolutions. The lines on the front side consisted of printed silver pasteand were an average of about 82 microns width as measured by a top-downoptical microscope. The busbars on the backside consisted of printedsilver paste and were a thickness of about 4.5 microns as measured byX-ray fluorescence (XRF).

The plating solutions were heated to 45° C. in a clear glass beaker.Solar cell pieces were immersed in plating solution for 8 minutes whileirradiating the front side using a 250 W halogen lamp from a distance ofabout 5 inches. The cells were then rinsed with deionized water anddried. The post-processing line widths were measured by a top-downoptical microscope and the backside busbar thicknesses were measured bXRF.

Table 1 shows the results for front side line width and backside busbarthickness measurements using optical microscopy and XRF respectively.

TABLE 1 Front side line width Backside busbar Plating solution (microns)thickness (microns) None (control) 82 4.5 Example 1 117 5.1 Example 2 985.6 Example 3 92 3.8 Comparative Example 1 81 2.8

It is observed that there are large increases in front side line widthsfor Examples 1-3, while Comparative Example 1 shows no increase. Inaddition, there is a net increase in backside silver busbar thicknessfor Examples 1-3, while Comparative Example 1 shows a net decrease,indicating that anodic corrosion of the busbar occurs.

Example 4

Comparison was made between a cell plated with silver from a commercialnon-cyanide silver electroplating solution (EPI E-Brite 50/50) and thecurrent invention.

A solution of this invention was made as follows:

-   -   17.5 grams/liter silver acetate    -   52.5 grams/liter hydantoin    -   22.8 grams/liter boric acid    -   8.7 grams/liter methanesulfonic acid    -   39.5 grams/liter Rochelle salt    -   52.0 grams/liter glyoxylic acid hydrate    -   KOH to was added to adjust the pH to 8.8

Solar cell pieces were plated with these solutions. The lines on thefront side consisted of printed silver paste and were an average ofabout 82 microns width as measured by a top-down optical microscope. Acell piece was plated in the solution of this Example 4 for 8 minutes at45 C in a clear glass beaker while irradiated the front side using a 250W lamp from a distance of about 5 inches. For comparison, a cell piecewas electroplated in the commercial silver plating bath by clamping asilver wire to the front side silver paste grid, which was thenconnected to a power supply with inert anode to complete the circuit,and passing a current equivalent to 0.4 amps/dm² for 180 seconds. Thecell pieces were then rinsed with deionized water and dried. Thepost-processing line widths were measured by a top-down opticalmicroscope.

The average finger width for the un-processed control was about 120microns as seen by topdown optical microscope. After plating the averagefinger width was about 130 microns for both the electroplated and lightinduced plated fingers. The surface profile of the finger plated usingthe current invention is smoother and more uniform, and the layer ofdense deposited silver is thicker for the current invention.

Example 5

Comparison was made of the back side silver paste busbars plated bylight induced plating, with and without the inclusion of Rochelle salt.

Solution 5A was made as follows:

-   -   13.5 grams/liter silver sulfate    -   89.6 grams/liter 5,5-dimethylhydantoin.    -   36.3 grams/liter sodium acetate    -   KOH to was added to adjust the pH to 9.3

Solution 5B was made identical to solution 5A, except 10.75 grams/literRochelle salt was included.

Solar cell pieces were plated with these solutions. Cell pieces wereplated in solutions 5A and 5B for 7 minutes at 50 C in a clear glassbeaker while irradiating the front side using a 250 W lamp from adistance of about 5 inches. The cell pieces were then rinsed withdeionized water and dried.

Severe corrosion of the silver paste comprising the back side busbar iseasily observable when plated in solution 5A without the inclusion ofRochelle salt, while no corrosion is visually observable when plated insolution 5B, with the inclusion of Rochelle salt. The thickness ofsilver in the bus bars was measured by X-ray fluorescence (XRF). For thepiece plated in solution 5A non-uniform thicknesses ranging from about0.1 to 2.7 microns were measured. For the piece plated in solution 5B, auniform thickness of 3.5 microns was measured.

Example 6

Comparison was made of both the front side silver paste fingers and theback side silver paste busbars after light induced plating, with andwithout the inclusion of citric acid or Rochelle salt.

Solution 6A was made as follows:

-   -   8.6 grams/liter silver sulfate    -   54.0 grams/liter 5,5-dimethylhydantoin    -   19.0 grams/liter acetic acid    -   KOH to was added to adjust the pH to 9.2

Solutions 6B-6H were made identical to solution 6A, except that thesolubilizers shown in Table 2 were included.

TABLE 2 Plating Solubilizer concentration solution Solubilizer(grams/liter) 6A none n.a. 6B Rochelle salt 12.0 6C Citric acid 15.0 6DSulfamic acid 23.0 6E Boric acid 16.4 6F Disodium phosphate 12.0 6GSodium glucoheptanoate 12.8 6H Glycine 14.2

Solar cell pieces were plated with these solutions. Cell pieces of about5×5 cm² were plated in solutions 6A-6H for 6 minutes and 16 minutes at50 C in a clear glass beaker while irradiating the front side using a250 W lamp from a distance of about 5 inches. The cell pieces were thenrinsed with de-ionized water and dried. Cells were weighed before andafter plating to determine the amount of mass gained. Thepost-processing line widths were measured by a top-down opticalmicroscope and the backside busbar thicknesses were measured by XRF. Theun-processed (control) cell had a front side finger width of 94 micronsand a back side silver busbar thickness of 7.7 microns.

TABLE 3 Front side silver Back side silver Cell mass gain finger widthbusbar thickness Plating (milligrams) (microns) (microns) solution 6min. 16 min. 6 min. 16 min. 6 min. 16 min. 6A 4 7 106 115 7.3 7.1 6B 1151 114 136 7.9 9.0 6C 12 56 108 138 7.8 9.6 6D 2 18 103 115 6.9 6.6 6E−1 7 94 104 5.8 5.6 6F 1 2 104 107 6.4 2.9 6G 3 11 110 126 7.2 6.8 6H 79 99 108 6.4 6.1

It is seen that light induced plating solutions 6B and 6C, containingRochelle salt and citric acid respectively, gave the largest mass gainsand finger widths after plating.

What is claimed is:
 1. A method of metallizing a photovoltaic solar cellto deposit a layer of metal thereon, said photovoltaic solar cell havinga front side and a backside, wherein said back side comprises a layer ofmetal selected from the group consisting of aluminum and aluminum alloysand said front side comprising a pattern comprising metal thereon, themethod comprising the steps of: a) contacting the photovoltaic solarcell with a light induced plating composition comprising: i) a source ofsoluble silver ions; and ii) an agent that solubilizes aluminum metalions dissolved from the layer of metal on the back side; and thereafterb) illuminating the photovoltaic solar cell with radiant energy from alight source, wherein metal ions from the light induced plating solutionare plated onto the metallic pattern on the frontside of the solar cell,and wherein the solar cell is not electrically connected to an externalpower source during the plating, and wherein ions of the metal layer onthe back side are dissolved into the light induced plating composition.2. The method according to claim 1, wherein the light induced platingcomposition comprises a complexing agent for the silver ions.
 3. Themethod according to claim 2, wherein the complexing agent is selectedfrom the group consisting of cyanide, succinimide or substitutedsuccinimide, hydantoin or substituted hydantoin, uracil, thiosulfate,amines and combinations of one or more of the foregoing.
 4. The methodaccording to claim 3, wherein the complexing agent is hydantoin orsubstituted hydantoin.
 5. The method according to claim 2, wherein theconcentration of the complexing agents is between about 20 to about 150grams/liter.
 6. The method according to claim 1, wherein the source ofsoluble silver ions is selected from the group consisting of silveroxide, silver nitrate, silver methanesulfonate, silver acetate, silvercitrate, silver sulfate and combinations of one or more of theforegoing.
 7. The method according to claim 6, wherein the source ofsoluble silver ions is silver acetate.
 8. The method according to claim6, wherein the source of soluble silver ions is silver methanesulfonate.9. The method according to claim 6, wherein the source of soluble silverions is silver sulfate.
 10. The method according to claim 1, wherein theconcentration of soluble silver ions is between about 1 and about 35grams/liter.
 11. The method according to claim 1, wherein thesolubilizing agent is selected from the group consisting of glyoxylicacid, glycolic acid, lactic acid, any alpha- or ortho-hydroxycarboxylicacids, aliphatic dicarboxylic acids, hydroxyl-polycarboxylic acids, orany combinations of the foregoing.
 12. The method according to claim 11,wherein the solubilizing agent comprises tartaric acid or salts thereof.13. The method according to claim 11, wherein the solubilizing agentcomprises citric acid or salts thereof.
 14. The method according toclaim 1, wherein the concentration of the solubilizing agent is betweenabout 1 to 150 grams/liter, but is most preferably from 10 to 30grams/liter.
 15. The method according to claim 1, wherein the patterncomprising metal on the front side of the solar cell comprises currentcollection lines and busbars printed thereon.
 16. The method accordingto claim 15, wherein the pattern comprising metal comprises printedsilver paste.
 17. The method according to claim 1, wherein the lightsource is selected from the group consisting of halogen lamps,incandescent lamps, fluorescent lamps, light emitting diodes, or mercurylamps.
 18. The method according to claim 1, wherein the step ofcontacting the photovoltaic cell with the light induced platingcomposition comprises immersing the photovoltaic cell in the lightinduced plating composition.